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Publishing Venue

The IP.com Prior Art Database

Related People

Kevin H. Grace: INVENTOR

Abstract

The use of unscheduled contention based channel access protocols in mobile ad hoc networks is very attractive due to their transparency to toplolgical change, relatively low overhead, and ease of implementation. Recent reports [1], however, have shown that the wodely available asynchronous contention based 802.11 protocol has significant fairness issues especially with regard to exposed terminals. These fairness issues are a direct result of the asynchronous nature of the protocol. In this paper, we present the Synchronous Unscheduled Multiple Access protocol which has all the benefits of a contention based access protocol while insuring fair access to all nodes

Country

United States

Language

English (United States)

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The use of unscheduled contention based channel access protocols in mobile ad hoc networks is very attractive due to their transparency to topological change, relatively low overhead, and ease of implementation. Recent reports[1], however, have shown that the widely available asynchronous contention based 802.11 protocol has significant fairness issues especially with regard to exposed terminals. These fairness issues are a direct result of the asynchronous nature of the protocol. In this paper we present the Synchronous Unscheduled Multiple Access protocol which has all the benefits of a contention based access protocol while insuring fair access to all nodes.

1. Introduction

Coordinating when nodes may transmit, ie. channel access, in a distributed manner can be accomplished in variety of ways. With many different proposed and existing access protocols, it is helpful to classify protocols into categories with related behaviors, namely: unscheduled asynchronous protocols, unscheduled synchronous protocols, statically scheduled protocols and dynamically scheduled protocols. Any protocol that is scheduled implies that it is also synchronous.

ALOHA, CSMA, and 802.11 Distributed Coordination Function (DCF)[2] are examples of unscheduled asynchronous protocols. While they differ in complexity, each operates without any kind of schedule for when to transmit, and nodes' clocks are not synchronized. In the case of ALOHA, nodes transmit as soon as they have a packet to send. This simplistic approach performs poorly except under very light loads. CSMA attempts to improve upon the ALOHA approach by sensing the channel prior to transmitting and deferring unless the channel is idle. While this is an improvement, CSMA suffers from the well known "hidden" terminal problem and does not perform well in a mobile ad hoc environment. 802.11 DCF augments physical channel sensing with a virtual carrier sensing mechanism to overcome the well known hidden terminal problem. And while this is an improvement over CSMA,

it suffers from the exposed terminal problem. It is this vulnerability that makes synchronous access protocols attractive for ad hoc access.

TDMA and TSMA[3] are examples of statically scheduled synchronous protocols. In the case of simple TDMA, a priori knowledge of all nodes enables a frame to be created with at least a single time slot for each node. While this approach guarantees collision free access to the channel for every node, it is wasteful when nodes do not utilize their allocated slots, precludes spatially reusing time slots, and imposes larger than necessary delays under light and moderate loading. TSMA attempts to improve upon TDMA by allowing a node to transmit in multiple slots in a frame but only guaranteeing that one of the slots will be contention free. While this can reduce...